High-frequency transformer



Dec, 27, 1932.

W. A. MaCDONALD HIGH FREQUENCY TRANSFORMER Filed NOV. 28, 1930 INVENTOR W/Y//am Mafa/m /d ATTORNEYS i Patented Dec. 27, 1932 UNITED STATES PATENT ,oFFlcE I l WILLIAM A. MACDONALD, OF LITTLE NECK, NEW YORK, ASSIGNOR T HAZELTINE COR- PORATION, OF JERSEY CITY, NEW JERSEY, A CORPORATION OF DELAWARE HIGH-FREQUENCY TRANSFORMER Application led November 28, 1930, Serial No. 498,785, and in Australia September 26, 1928.

This invention relates to high-frequency transformers and more particularly to transformers of this'type whichv are utilized asy antenna circuit is resonant at a frequency somewhat below the tuning range, whereby the antenna circuit is rendered inductively reactive throughout the tuning range of frequency. This is accomplished by winding the primary, or'antenna, winding of the transformer with a sufficiently large number of turns so that the inductance of the winding resonates with the antenna capacity at the desired low frequency. The secondary winding is tuned tothe signal frequency by means of a variable condenser connectedacross. the secondary winding. The coupling between the primary and the secondary windings is electromagnetic or electrostatic or a combination of c ach. Tf both types of coupling are employed together, they may either mutually aid or oppose depending upon whether itis desired to emphasize the higher or the lower frequencies of the tuning range. The abovementioned co-pending application discusses in detail the advantages of an inductive antenna circuit.

The use of an inductive reactive antenna system offers important advantages over the usual capacitive antenna system employed in broadcast reception. One of the important advantages resides in the improved alignment of the several tuned circuits while a second important advantage resides in the fact that vthe amplification characteristic may be made more nearly uniform or, in fact, may be modified over a wide range.

As regards the first advantage, when the ordinary capacitive antenna is employed, the` antenna capacity is refiected through the antenna coupling transformer to the tuned secondary circuit where it has the effect of a capacity in shunt with the tuning condenser. This condition is undesirable for it then becomes necessary to provide additional paddin capacities across the tuning condensers of t e following stages or requires an adjustable tuning capacity in the input circuit. In the case of the inductive antenna circuit, on the other hand, the effect of the antenna is to reduce the measured secondary inductance by a constant so that the effective inductance of the secondary becomes slightly less than its original value, measured in the absence of the primary circuit. So this reduction can easily be vcompensated by adding a few more turns to the secondary winding, whichis easy to accomplish because the inductance is not the tuning element but is fixed.

As regards the second advantage, namely, of obtaining uniformity of amplification, it is well known thatl the usual capacitive antenna discriminates in favor of the higher frequencies of the tuning range. This discrimination is usually greatly accentuated by thev fact that most interstage coupling systems also discriminate in favor of the higher frequencies. Hence, the overall radio-frequency gain of the receiver may be very high at the high-frequency range and fall to a very low value at the low-frequency range. When the antenna circuit is resonant at a frequency slightly below the tuning range, it presents the lowest seriesy impedance to the Alower frequencies of the range and hence discriminatesin favor of the lower frequencies, thereby compensating forthe amplification characteristic of the interstage coupling systems. I

Control over the amplification characteristic may be further exercised by utilizing the interwinding capacities to supplement the magnetic coupling. This interwinding capacity may be' arranged to either aid or oppose the magnetic coupling. Whether it aids or opposes depends upon the relative directions of winding the primary and secondary windings. The capacity coupling obviously increases with frequency. If, therefore, it is arranged to aid the magnetic coupling, it

will tend to accentuate the higher frequencies 100 more than the lower; but if it is arranged to oppose the magnetic coupling, it will tend to accentuate the lower frequencies more than the higher. Interwinding capacity may be made use of in the same manner in the case of the interstage coupling transformers.

The present invention is directed to the particular mechanical construction of transformers which are especially well adapted to perform the functions described above.

Features of the invention are the relative positioning, poling and proportioning of the primary and secondary windings and their associated terminals to provide an efficient, simple and inexpensive transformer structure.

The transformers of this invention are adapted to couple the stages of an amplifier as well as to couple the antenna to the first amplifier stage.

The details of construction will be better understood by reference to the accompanying drawing and the description pertaining thereto.

Fig. l shows a transformer constructed in accordance with this invention, the only substantial coupling between the primary and secondarywindings being electromagnetic;

Fig. 2 shows an alternative construction of an electromagnetically coupled transformer;

Fig. 3 illustrates a transformer designed to have electrostatic as well as electromagnetic coupling;

Fig. 4 illustrates another form of transformer in which primary and secondary windings are coupled electromagnetically and electrostatically;

Fig. 5 illustrates a manner of providing any desired amount of capacity coupling between primary and secondary windings.

Fig. 1 illustrates in section a transformer constructed in accordance with this invention. The coil structure comprises a cylindrical form 20 which is preferablv of impregnated cardboard, bakelite or other suitable insulating material which supports a winding 21 which is the secondary winding of the transformer. This winding may be composed of enameled wire suitably spaced between turns, or a more heavily insulated wire of which adjacent turns are close together. Terminal lugs are provided at both ends of the form and suitably fastened to the inner cylindrical surface thereof to facilitate connections to the coil terminals, terminal lug 22/at one end of the form being the high-potential terminal of the secondary, and terminal 23 at the opposite end being the low-potential terminal. A bobbin 24 which is located within the cylindrical form at the low-potential end of the secondary winding is composed of an insulating material such as wood, bakelite or hard rubber pressed tightly within the supporting form 20. The bobbin is provided with a narrow circumferential slot 25 in which a primary coil 26 is wound. The primary winding consists of turns of insulated wire, random-wound in the slot. Terminal lugs 27 and 28 are provided at the low-potential end of the supporting form 2O to serve as connecting terminals for the ends of the primary winding, lug 27 be ing the high-potential terminal of the primary.

A metal can 29, preferably of copper or aluminum or other metal having low specific resistance, encloses the complete transformer construction, the purpose being to restrain the electrostatic and electromagnetic fields from effecting undesirable coupling with other parts of the system. There is provided at the top of the shielding can a small hole 38. through which the high-potential lead of the secondary winding may be led.

In this embodiment of the invention special consideration is given to the location of the bobbin containing the primary winding and the disposition of the terminal lugs of the several coil ends. Itis desired, in this embodiment, to have only magnetic coupling between the primary and secondary windings with out appreciable capacitive coupling. To effect this result care must be taken to minimize the capacity which would normally exist between the primary and secondary windings and their associatedterminals. With this in View, the bobbin containing the primary winding is located at the low-potential end of the secondary winding and the high-potential terminal lugs of the two windings are located at opposite 4ends of the cylindrical form. By virtue of this construction the capacity coupling due to the capacity between terminal lugs 22 and 27 is reduced to a negligible amount and the capacity between the high potential point of winding 21 and the high-potential end of winding 26 is reduced to a minimum. The primary winding is wound in such a direction with respect to the direction of the secondary winding that the high-potential primary terminal is located at the outside of the bobbin, that is, close to the low-potential end of the secondary winding. By this arrangement of winding, the low-potential end of the secondary coil, which is at or near ground potential, serves as a partial electrostatic shield which tends to further minimize capacity coupling between the two windings.

It should be understood that `the terms high potential and low potential as employed in this specification refer to the signal potentials. By a low potential terminal is meant one which is at, or near, the signal potential of an associated amplifier cathode, that is, near ground potential. A high potential7 terminal is one which is removed from ground potential, such as a terminal to be connected to a grid or an anode'or an an! tenna.

The entire construction comprising the supporting form with its coil windings and the shielding can may be mounted on the metal chassis 39 of the radio receiver or other suitable mounting means having apertures sufficiently large to pass the ends of the terminal lugs 23,27 and 28 without short circuit; or the lower ends of the transformer and shielding can may be left entirely open and the structure supported by means of a bracket. In the latter case the can 29 may be fitted to the chassis by spinning it into a hole of proper size or by bolting.

To enable the invention to be readily practiced, there is given the following specification of a transformer construction which has proved very satisfactory Inches Outside diameter of cylindrical form for secondary winding 1% inside diameter of cylindrical form for secondarywinding 11/8 Outside diameter of primary bobbin 11/8 Inside diameter of slot in primary bobbin 5/8 lidth of slot in primary bobbin 1/8 Diameter of cylindrical shielding can 2% Primary winding 200 turns #36 double silk covered copper wire, random wound. (This will give an inductance of the order of 600 microhenries.) Secondary winding 120 turns of .#:30 enameled copper wire,

closely wound in a single layer. (This will give an inductance of the order of 250 microhenries.) lt has been found to be often advisable, when using the transformer as an interstage instead of an antenna coupling transformer, to wind more than the two hundred primary turns mentioned above; four hundred to eight hundred primary turns (the primary inductance varying from about two to ten millihenries) have been found well suited for this purpose.

The above table of values should not be construed as a limitation upon the invention, but is given merely to indicate proportions which have been found satisfactory.

lt should be noted that thel structural dimensions of the transformers in the drawing are approximatelyin proportion to the above dimensions and are drawn approximately full size. l

It has been found from experience that the most efficient types of windings for the two coils are approXimately those shown in the drawing; that is to say, the first winding, referred to as the secondary, is helical in form and the length of winding is equal to or longer than its diameter. The second winding, referred to as the primary, has been found to be most efficient when wound in very concen- 'interstage transformer.

trated form so that its length is less than its diameter. l

The transformers of this inventionare as well adapted ,for us'e as interstage couplings as for antenna couplings. The desirability of rendering the antenna circuit inductive over the tuning range of frequency has been explained. But it is also desirable from the standpoint of good amplifier operation that the impedance connect-ed in the output circuits of the amplifier tubes shall be capacitively reactive over the same tuning range. These two considerations can both be obtained by the same transformer construction, that is, by the provision of a relatively large number of primary turns. That this is so follows naturally from the fact that the antenna capacit* is in series with its associated primary win ing, whereas the capacities associated with the output of an amplifier are in shunt with the interstage primary winding. It is Well lrnown that a relatively large inductance in a series resonant circuit renders the' circuit inductive above a relatively low frequency; and a relatively large inductance in a parallel resonant circuit renders the circuit capacitive above a relatively low frequency. Hence the same transformer is adapted to serve either of the two purposes, but as already mentioned it is generally necessary to increase the number of turns on the primary winding when employing it as an The reason for this change is that the capacity of the output device to which it is connected is usually much smaller than the antenna capacity, so to have the primary circuits throughout the amplifier tuned to the same order of frequencies it is necessary to increase this primary inductance.

Fig. 2 illustrates a transformer construc- 'f tion somewhat similar to that of Fig. l. In the case of this transformer, as in that of Fig. l, it is desired that the primary and secondary windings be coupled only magnetically and not capacitively. In the structure of Fig. 2 the disposition of the primary and secondary coils is similar to that of Fig. l, but the terrminal lugs are all arranged at one end of the (ylindrical form and in such a manner that the high potential points arerseparated as far as possible. In attaining this result, one of the terminal lugs 30 which is at or near ground potential is interposed between the high potenti'lterminal lugs 3l and 32 of the primary and secondary windings, respectively; this further reduces capacity between the windings. The high potential end of the secondary winding is connected to terminal lug 32 at the lower end of the form which is nearest the primary bobbin instead of at the high potential end of the secondary winding as in Fig. l. To enable this connection to be made a longitudinal passage 33 is provided through the bobbin through which the high potential secondary lead is passed. The low potential ends of both windings are shown connected to the low potential lug 30. In the construction ot Fig. l, the relative directions of the primary and secondary windings is reversed with respect to directions of winding in Fig. l, so that the high potential terminal of the primary winding is located at the inside of the bobbin and the low potential primary terminal is at the outside of the bobbin.

The arrangement of 2 is simply an alternative of that of Fig. 1, and is sometimes preferred because of the simplified terminal and circuit connections and also because of the fact that it is not always convenient to locate the high potential secondary terminal at the top of the coil.

Fig. 3 shows a transformer arrangement in which it is desired to have the primary and secondary windings coupled both electrostatically and electromagnetically; only a limited amount of electrostatic coupling is desired, however. This small amount of capacitive coupling in addition to the magnetic coupling is obtained by arranging the windings in a manner similar to that shown v in Fig. l, and by so positioning the terminal lugs that there exists substantial capacity between the high potential terminals of the two windings. To produce this effect the high potential terminals 4l and 42 of the primary and secondary windings, respectively, are arranged adjacent to each other.

The capacity and magnetic couplings may e so arranged that they mutually aid or oppose, depending upon the type of response characteristic desired. This mutual opposition or addition of electrostatic and electromagnetic coupling is provided by regulating the relative direction of winding the primary and secondary coils or by the method of connecting the coils. If, for example, it is desired that the capacitive coupling aid the magnetic coupling, the windings may be wound in opposite directions and may beso arranged that the'low potential primary terminal and the low potential secondary terminal would be connected to the same'terminal lug. If, on the other hand, it is desired thatl the previous case. Y

Fig. 4 illustrates a form of the invention in which the primary bobbin 50 is located at the high potential end of the secondary winding. This arrangement is adapted to rovide a substantial alnount of electrostatic as well as electromagnetic coupling. In order that the electrostatic coupling shall be as large as possible, the primary coil is so wound on the bobbin that its high potential end 5l is at the outside of the bobbin, ,that is, very close to the high potential turns of the secondary winding. The high potential terminal lugs of the primary and secondary windings are numbered, respectively, 52 and 53, and the low potential terminal lugs are respectively numbered 54 and 55. If it is desired that the capacity coupling aid the magnetic coupling, the two coils should be wound in opposite directions when considering the low potential ends of the windings as the starting points.`

It has been found preferable to employ in transformer structures of this type a primary winding consisting of a relatively large number of turns of line wire wound in a very flat bobbin, that is, a bobbin having a very small axial length. This form of winding is desirable because there is present a minimum amount of distributed capacity, thus giving the primary wind-ing a high inductive reactance which it has been pointed out is desirable in the case of antenna coupling trans formers. The invention, however, is not limited to the use of a bobbin for supporting the primary winding; it may be more convenient, for example, to employ as the primary winding a self-supporting coil of the so-called universal type, which may be supported by a small mandrel approximately in the position shown in the figures. Furthermore, it is not essential that the primary winding be located inside the secondary; it may be located in any position adjacent the secondary, Jfor example, beyond the end of the secondary winding.

In some cases, and particularly where a large amount of capacity coupling is required, it may be desirable to make the primary coil somewhat longer than the accompanying illustrations indicate, since a longer coil exposes more high'potential area and hence has greater inter-winding capacity. y

It has been found convenient in arriving at the desired amount of magnetic and of capacitive coupling to proportion the. depth ofthe slot in the primary bobbin, so that the primary coil is located the proper distance inside the secondary winding. Assume. for example, that a transformer is desired having a certain mutual inductance between the primary and secondary, and that this mutual inductance is obtained by constructing the primary winding with a certain diameter within the secondary coil. Very often it happens that when this desired mutual inductance is obtained, the capacity coupling bein a single layer as illustrated in the drawing; the secondaries may be equally well constructed, for example, as a bank-wound coil, that is, a double wound coil of which the turns of the outer layer rest evenly in the spaces between the turns of the inner layer.

There should also be noted the possibility of obtaining capacity coupling without any appreciable magnetic coupling, although this latter condition is not frequently desirable. lt can be attained, however, by orienting the primary coil so that its axis is at right angles to the aXis of the secondary coil.

The transformers of this invention will ordinarily be used with a shield as shown in the drawing. The use of the shield is particularly important where there is desired a certain small amount of capacity coupling between the windings, and also where no capacity coupling at all is desired, for under these conditions, the presence of electrostatic fields from external sources would seriously affect the ldesired results.

What is claimed is:

1. A radio-frequency transformer comprising a secondary winding having a length greater than its diameter and a primary winding located within said secondary winding, said primary winding having an axial length which is small in comparison with that of said secondary winding, the number of turns on said primary winding being greater than the number of turns on said secondary winding.

2. A radio-frequency transformer comprising an evenly wound secondary winding and a multi-layer primary winding, said secondary winding being wound cylindrical in form and said primary winding being wound concentrically Within said secondary, said primary winding having an inductance which is greater than the inductance of said secondary winding and being wound so that the high potential terminal thereof is the terminal of said primary winding which is at its outer circumference.

3. A high-frequency transformer comprising a helical-wound secondary coil and a closely wound primary coil, said primary coil being situated co-aXially within said secondary coil and at one end thereof, the high potential end of said primary coil being adjacent the low potential end of said secondary coil, whereby the coupling between said coils is substantially entirely magnetic, and a metallic shield at least partially enclosing said transformer for minimizing undesirable coupling with external sources.

4. A radio-frequency transformer comprising a helical-Wound secondary winding having a length at least as great as its diameter, and a primary winding of small axial length relative to that of said secondary winding located adjacent one end of said secondary a larger number of turns than said secondary Ywinding whereby a circuit in which said primary winding is connected is rendered resonant at a frequency which is below the range of frequency to be received by said circuit.

5. A radio-frequency transformer according to claim 4 in which said primary coil is located at the low-potential end of said secondary coil and the high-potential end 0f said primary coil is at its outer circumference, whereby the capacitive coupling between the windings is negligible.

6. A radio-frequency transformer comprising a cylindrical core, a secondary winding wound in one layer on said core, a bobbin and a primary winding wound in a circumferential slot in said bobbin, said bobbin being located concentrically within said core at one end of said secondary winding and said slot having an axial length which is small in comparison with the axial length of said layer of secondary winding, the number of turns of said primary winding being larger than the number of turns of said secondary winding whereby a circuit in which said primary winding is connected is resonant at a lov7 radio frequency.

7. A radio-frequency transformer according to claim 6 having terminal lugs for the ends of the coils, of which said primary coil is located at the low-potential end of said secondary coil and the high-potential end of said primary coil is at the outside 0f said bobbin and the terminal lug to which the high-potential end of the secondary winding is connected is located at the end of said form nearest the high-potential end of the second'- ary coil and the terminal lugs of the primary coil are located at the opposite 'ends of the form, whereby the capacitive coupling between the coils is negligible.

8. A radio-frequency transformer comprising a cylindrical form, a secondary coil helically wound on said form, a multi-layer primary coil located co-axially within said secondary coil and located at the low-potential end of said secondary coil and terminal lugs for the ends of said coils, said primary coil being so wound that the low-potential end thereof is situated at the outer circumference of said primary coil, the high-potential termiferential slot of said bobbin, said bobbin being fastened axially within said form and at the low-potential end of said secondary winding and said primary winding being so wound on said bobbin that the low-potential end thereof is situated at the outer circumference of said bobbin.

10. A high-frequency transformer comprising a helical-wound secondary coil, a closely wound multi-layer primary coil, and terminal lugs for the ends of said coils, said primary coil being situated co-axially with respect to said secondary coil and near the lowpotential end thereof, said coils being wound in opposite relative directions, and the highpotential terminal lugs of said coils being located adjacent each other, whereby said coils are coupled magnetically and slightly capacitively.

11. A high-frequency transformer comprising a helical-wound secondary coil and a vclosely wound multi-layer primary coil, said primary coil being located near the highpotential end of said secondary coil, the portion of said primary coil which is nearest the secondary coil being the high-potential portion, whereby the coupling between said coils is substantially capacitive as well as magnetic.

12. A high-frequency transformer according to claim 11 in which the effective direction of said primary winding is opposite that of said secondary winding, whereby said capicitive coupling aids said magnetic cou- 13. A high-frequency transformer according to claim 11 in which said primary coil is effectively wound in the same direction 'as said secondary winding, whereby said capacitive coupling opposes said magnetic coupling.

-l/l. A transformer comprising two windings, the first of said windings being cylindrical in form and having an axial length at least equal to its diameter, the second of said windings being a multi-layer winding and having an axial length small in comparison to its diameter, said second winding comprising a relatively large number of turns in comparison with said first winding and being supported near one end of said first winding, said windings being coupled electromagnetically and electrostatically, whereby said system may be adapted to have a predetermined transmission characteristic, and a metal cylinder of l'ow specific resistance circumferentially surrounding said windings.

l5. A coupling system comprising two windings, the first of said windings being a cylindrical coil having an axial length at least as great as its diameter, the second of said windings being a multi-layer coil having an axial length small in comparison with its diameter, the number of turns of said first coil being small in comparison with the number of turns of said second coil, the high ponagaan tential points of said coils being electrostatically coupled, and a metal cylinder of low specific resistance surrounding said coils.

16. A transformer comprising two windings, the first of said windings being cylindrical in form and having an axial length at least equal to its diameter, the second of said windings being a multi-layer winding and having an axial length small in comparison to its diameter, said second winding comprising a relatively large number of turns in comparison with said first winding and being supported within one end of said first winding, said windings being coupled electromagnetically and electrostatically,whereby said system may be adapted to have a predetermined transmission characteristic, and a metal cylinder of low specific resistance circumferentially surrounding said windings. `l7. A coupling system comprising two windings, the first of said windings being a cylindrical coil having an axial length at least as great as its diameter, the second of said windings being a multi-layer coil having an axial length small in comparison with its diameter, the number of turns of said first coil being small in comparison with the number of turns of said second coil, the high potential points of said coils being electrostatically coupled. c

18. A radio-frequency transformer comprising a helical-wound secondary winding having a length at least as great as its diameter, and a primary winding of small axial length relative to that of said secondary winding located within one end of said secondary winding, said primary winding comprising a larger number of turns than said secondary winding, whereby a circuit in which said primary winding is connected is rendered resonant at a frequency which is below the range of frequency to be received by said circuit.

In testimony whereof I affix my signature.

WILLIAM A. MACDONALD.

DISCLAIMER 1,892,354.-William A. MacDonald, Little Neck, N. Y. HIGH-FREQUENCYTRANS- FORMER. Patent dated December 27, 1932. Disclaimer' filed January 15, 1934, by the patentee, the assignee, Hazeli-no Corporation, consenting.

Therefore, enters this disclaimer to claims 4, 5, 11, 12, 13, and 17, to Wit:

He disclaims from claim 4:

A radio-frequency transformer' comprising a helical-wound secondary winding having a length at least as great as its diameter, and a primary winding of small axial length relative to that of said secondary winding located adjacent one end of said secondary Windingi except- Aradioefrequency transformer comprising a helical-wound secondary winding having a length at least as great as its diameter, and a primary winding of small axial length relative to that of said secondary7 winding located adjacent one end of said secondary Winding, having a. metal shield of low specific resistance circumferentially surrounding said windings.

He disclaims from claim 5:

A radio-frequency transformer according to claim 4 except- A radio-frequency transformer according to claim 4 having a metal shield of low specific resistance surrounding the windings of said transformer.

He disclaims from claim 11:

A high frequency transformer comprising a helical-wound secondary coil and a closely wound multi-layer primary coilexceptl A high frequency transformer comprising a helical-wound secondary coil and a closely Wound multilayer primary coil having a metal shield of low specific resistance surrounding said coils.

He disclaims from claim 12:

A high frequency transformer according to claim 11 except- A high frequency transformer accordingr to claim 11 having a metal shield vof low specific resistance surrounding the coils of said transformer.'

He disclaims from claim 18:

A high frequency transformer according to claim 11 except- .A high frequency transformer according to claim 11 having a metal shield of low specific resistance surrounding the coils of said transformer.

He disclaims from claim 17:

A coupling system comprising two windings except- A coupling system comprising two windings having a metal shield of low specific resistance surrounding said windings.

[Official GazetteJanuam/ 30. 798.41 

